Sedimentology and paleontology of an upper Jurassic-lower cretaceous shelf to alluvial plain coal-bearing sequence, northwest British Columbia, Canada
Strata of the upper Bowser Lake Group in the northern Bowser Basin includes Oxfordian to Early Cretaceous, pre-Albian elastics which were interpreted to have been deposited in a shallow shelf to upper delta plain and alluvial fan setting on the basis of facies relationships, trace fossils, marine, brackish and fresh water faunas, plant macrofossil assemblages, coal chemistry and siderite and pyrite distribution. The strata studied include, in ascending order, the Jackson unit, and the Currier, McEvoy and Devils Claw formations. The lowermost Jackson unit is a record of storm-dominated shoreface and foreshore deposition in a high energy, largely gravel-dominated shoreline during Oxfordian to Tithonian time. Bioturbation is minimal. Environmentally diagnostic sedimentary structures in the 1-20 m thick coarsening-upwards sequences include hummocky and low angle planar cross-stratification. The occurrence of dispersed pebbles, bivalve coquinas, shore-parallel large fossil logs and plant detritus increases upwards. The overlying, anthracite-bearing, Currier Formation is interpreted to have been deposited in an interdistributrury bay setting. This interpretation is based on a brackish water trace fossil assemblage of mixed Skolithos-Cruziana ichnofacies affinity, the macrofaunal content, coal chemistry and facies associations. Common facies in this environment include: 1) Macaronichnus-bearing shallow subtidal to intertidal beach sandstones; 2) trough cross-bedded fluvial and/or tidal channels; 3) crevasse splay deposits; 4) floodplain and lacustrine sediments with the fresh water bivalve Ferganoconcha and the ichnogenus cf. Cochlichnus; 5) coal; and 6) bioturbated, fine grained bay-fill sequences. Facies associations attest to the interplay between two dominant processes: 1) interdistributary bay filling by overbank flooding and crevasse splay processes; and 2) transgression by marine waters of fresh and brackish water peat swamps, probably caused by local subsidence. Fluvial channels, very thin coals, floodplain and crevasse splay facies of theoverlying non-marine McEvoy Formation pass upwards into conglomerate-dominated proximal braidplain and alluvial fan streamflow and debris flow facies of the Devils Claw Formation. The Kimmeridgian to Hauterivian unconformity, traditionally interpreted for the southern Bowser Basin, and recently extended into this area on the basis of palynology, is challenged on the basis of the sedimentological model as well as the faunal and floral assemblages. The lowermost Jackson unit contains a suite of biostratigraphically significant, Late Jurassic (mid Oxfordian to Tithonian) Buchia spp. and Vaugonia doroschini, recorded for the first time in the Groundhog coalfield. The overlying Currier, McEvoy and Devils Claw formations contain the Early Cretaceous (Neocomian) bivalve Herzogina, in addition to pre-Aptian plant macrofossil assemblages dominated by typically Jurassic and Early Cretaceous (Aptian and older) cycadophytes, ginkgophytes and fems.
Bibliography: p. 250-290.
MacLeod, S. E. (1991). Sedimentology and paleontology of an upper Jurassic-lower cretaceous shelf to alluvial plain coal-bearing sequence, northwest British Columbia, Canada (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/24444